This invention relates to an electrical waveform generator for driving electromechanical devices such as piezoelectric and magnetostrictive transducers.
Tools that are made to vibrate at frequencies above the range of human hearing have been used in science, medicine and industry for many decades. Applications such as cell disruption, bloodless surgery, welding of metals and plastics and sewage treatment are well known to the art.
Ultrasonic devices generally consist of a transducer, which converts electrical energy to mechanical vibration, a probe or horn which amplifies the vibration amplitude of the transducer and an electric or electronic signal generator which converts line or battery power to the AC signal with the frequency and voltage necessary to drive said transducer against the load imposed.
One of the elements of these systems is a limiting factor to greater use of ultrasound energy in the marketplace. This is the signal generator.
Transducers and probes are fairly easily designed and built to meet specific application requirements, such as use as a surgical tool or as a cell disruptor in biosciences. Each transducer and probe combination will have a characteristic resonance curve of impedance vs. frequency, as depicted in FIG. 1. The minimum impedance point IPMIN of the curve is generally regarded as the series resonance and the maximum impedance point IPMAX is considered the anti-resonance or parallel resonance of the transducer. This curve will be either broad (FIG. 1A) or narrow (FIG. 1B). It may also have secondary resonances SR superimposed upon the curve, as shown in FIG. 1C. The absolute value of impedances is also important, in that the dynamic range of the impedances may be quite high from the point where the transducer is unloaded to that where the transducer is fully loaded.
In addition to the impedance vs. frequency characteristic curve, the transducer will undergo a phase shift between voltage and current signals when the frequency is increased from below the resonant points to a point above the both resonances. This phase shift will start as a current leading (−90 degrees) through zero at series resonance, up to current lagging (+90 degrees) back through zero phase shift and down to current lagging. This characteristic phase shift may differ greatly from ideal as the system is loaded highly or if the initial design of the transducer and probe combination is not optimized.
The electronic generator must provide a frequency and voltage/current drive which will keep the transducer and probe vibrating at the desired resonance point against the loads imposed upon it by the application. Traditionally these generators have used phase-locked-loop (PLL) techniques and auto gain circuitry (AGC) well known to the art. In these systems, great compromises are generally made since one generator may not be designed to encompass all known variations of impedance or phase vs. frequency and power requirements that may be encountered in the field. For example, in some systems, the frequency difference between Series and Parallel Resonance may only be a few hertz. Standard PLL systems may require such a low loop gain to capture and lock onto these high Q devices that the system is unstable in normal operating modes. In other systems, the phase characteristic curve may be flattened to the point that the PLL may not recognize the phase shift at series or parallel resonance and therefore not be able to lock onto either when needed.
Therefore, each application may require a different generator design, which is costly and time consuming to engineer. Therefore, many applications go wanting for a solution since no entity is willing to expend the time and money to provide the hardware solution.
One of the reasons for this is that, in conventional electronic design, the hardware must be changed physically for each application. Here, the hardware of the PLL loop must be optimized for each individual application. In addition, the amplifier section of the generator is usually tailored to the maximum power requirement of the particular application at hand, to minimize size and cost. If the next application requires a higher power output, the amplifier is generally redesigned from scratch to achieve this higher output rating.